1. Field of the Invention
The invention relates to a slurry hydrocarbon synthesis process, which includes increasing the activity of fresh catalyst in-situ in the slurry liquid, during the synthesis reaction. More particularly the invention relates to a slurry Fischer-Tropsch hydrocarbon synthesis process for producing hydrocarbons from a synthesis gas, wherein the activity of fresh synthesis catalyst added to the slurry reactor is increased in-situ in the slurry during the synthesis reaction.
2. Background of the Invention
The slurry Fischer-Tropsch hydrocarbon synthesis process is now well known and documented, both in patents and in the technical literature. This process comprises passing a synthesis gas which comprises a mixture of H2 and CO, up into a reactive slurry comprising a hot, synthesized hydrocarbon liquid, in which a suitable, particulate Fischer-Tropsch type of catalyst is dispersed and suspended, at reaction conditions effective for the H2 and CO to react and form liquid hydrocarbons. The hydrocarbon liquid is continuously withdrawn from the reactor and upgraded by one or more upgrading steps, which include fractionation and conversion operations such as hydroconversion, in which a portion of the molecular structure of at least some of the hydrocarbon molecules is changed. The upgraded products may include, for example, a syncrude, various fuels and lubricating oil fractions and wax. During the synthesis reaction, the presence of reversibly deactivating catalyst species present in the synthesis gas, such as NH3 and HCN, causes a reduction in catalyst activity. This activity loss is reversible and may be restored in-situ in the reactor by rejuvenation, as is known and disclosed, for example, in U.S. Pat. Nos. 5,283,216; 5,811,363; 5,811,468; 5,817,702; 5,821,270; 5,844,005 and 5,958,986. Catalyst rejuvenation is achieved by passing hydrogen or a hydrogen treat gas into the bottom of a vertically oriented, hollow conduit or tube immersed in the slurry or external of the synthesis reactor in a separate vessel. This sets up a slurry circulation in the tube, in which the hydrogen gas contacts the catalyst particles in the slurry liquid, thereby rejuvenating them by restoring at least part of their activity. Some of the activity loss is permanent, which ultimately requires catalyst regeneration or replacement. One of the advantages of the slurry process, is the ability to add and remove catalyst without taking the reactor offline and thereby stopping the synthesis reaction. U.S. Pat. No. 5,292,705 discloses that the activity of a fresh Fischer-Tropsch type of hydrocarbon synthesis catalyst can be increased by treatment with hydrogen in a hydrocarbon liquid, including in-situ in the hydrocarbon synthesis slurry in the synthesis reactor. The hydrogen treatment for increasing the catalytic activity is preferably conducted in the absence of CO. Thus, in this process, if the catalyst activity is increased by treatment in-situ in the hydrocarbon synthesis slurry in the slurry synthesis reactor, the synthesis reactor is not producing hydrocarbons while the activity of the fresh catalyst is increased. It would be an improvement to the art if the activity of fresh catalyst added to the reactor could be increased while the reactor is producing hydrocarbons and without adversely effecting the selectivity or productivity of the synthesis reaction.
The present invention relates to a process for increasing the activity, and therefore the productivity, of a fresh hydrocarbon synthesis catalyst above its initial value, by contacting it with hydrogen or a hydrogen treat gas, insitu in the hydrocarbon synthesis slurry used for synthesizing hydrocarbons in the slurry Fischer-Tropsch hydrocarbon synthesis reactor, while the reactor is producing hydrocarbons from the synthesis gas feed. This is achieved by adding the fresh catalyst to the hydrocarbon synthesis slurry in, or external of, the synthesis reactor and circulating a portion of the slurry containing the fresh catalyst up through one or more catalyst activity increasing means, the interior of which is isolated from the slurry body in the synthesis reactor. Hydrogen or a hydrogen treat gas is passed into the means, in which it contacts the fresh catalyst particles in the flowing slurry and increases their activity. By fresh catalyst is meant one that is new and has not been used, or has been in use only for a relatively short period of time and has an activity of at least 85%, preferably at least 90% and more preferably at least 95% of the initial value of a new and unused catalyst. By initial value is meant the lined out activity level the fresh catalyst exhibits when first added to the slurry, extrapolated back to zero time. The activity is calculated based on the extent of CO conversion and, for a given catalyst, is effected by the space velocity, temperature, and partial pressure of the reactants and reaction products, as well as the hydrodynamics in the slurry reactor. Irrespective of whether the slurry reactor is operated as a dispersed or slumped bed, the mixing conditions in the slurry will typically be somewhere between the two theoretical conditions of plug flow and back mixed. In contrast to the process disclosed in U.S. Pat. No. 5,292,705, in which the synthesis reactor is off-line and not producing hydrocarbons while increasing the activity of the fresh catalyst in-situ in the slurry in the synthesis reactor, in the process of the invention the activity of the fresh catalyst is increased in-situ in the slurry in the synthesis reactor without interrupting or interfering with the hydrocarbon synthesis reaction. This is made possible, because the catalyst activity increasing zone, even if it is wholly immersed in the slurry body in the synthesis reactor, is isolated from it. The catalyst activity increasing process of the invention eliminates the need for (i) the addition of heat to the reactor, (ii) additional hydrogen capacity and (iii) compressors to deliver sufficient hydrogen to the reactor to maintain slurry circulation and catalyst dispersion in the slurry liquid while increasing the catalyst activity.
In the process of the invention, the fresh catalyst in the form of solid particles or slurried in an appropriate hydrocarbon liquid, is added to the hydrocarbon synthesis slurry. The hydrocarbon synthesis slurry containing the fresh catalyst is contacted with a hydrogen treat gas in one or more catalyst activity increasing zones, in which it contacts the fresh catalyst particles and increases their activity. The slurry containing the activity increased catalyst is then returned to the hydrocarbon synthesis slurry body (hereinafter xe2x80x9cslurry bodyxe2x80x9d) in the hydrocarbon synthesis reactor. In one embodiment, the fresh catalyst is added to the hydrocarbon synthesis slurry, by adding it to the slurry body in the reactor, with a portion of the slurry body, now containing the fresh catalyst, circulated from the slurry body up through and out of the one or more catalyst activity increasing zones and back into the slurry body. In another embodiment, the fresh catalyst is added to the hydrocarbon synthesis slurry in the one or more zones, as the hydrocarbon synthesis slurry and hydrogen treat gas pass up and through the zones, with the slurry containing the activity increased fresh catalyst then returned back to the slurry body in the synthesis reactor. Thus, the expression xe2x80x9cthe fresh catalyst is added to the hydrocarbon synthesis slurryxe2x80x9d is meant to include one or both of these two embodiments. By xe2x80x9chydrocarbon synthesis slurryxe2x80x9d is meant either the slurry body in the synthesis reactor, or slurry withdrawn from the slurry body with or without gas bubble removal prior to passing it into the one or more catalyst activity increasing zones or means. The hydrocarbon synthesis slurry comprising the slurry body in the synthesis reactor comprises catalyst particles and gas bubbles dispersed in a hydrocarbon slurry liquid.
The catalyst activity increasing means may comprise a hollow, elongated and substantially vertically oriented fluid conduit, such as a hollow tube, open at both ends for fluid to flow through, the interior of which comprises the activity increasing reaction zone. The one or more activity increasing reaction zones may be inside the synthesis reactor, external of the reactor and in fluid communication with the slurry body in the reactor, or both. Irrespective of which of these three modes is employed in the process of the invention, the hydrocarbon synthesis slurry and catalyst activity increasing hydrogen in the activity increasing reaction zone are isolated from the slurry body in the reactor. Slurry circulation up through the means is achieved by the lifting action of the hydrogen treat gas and, therefore, the activity increasing means may be viewed as a form of lift tube reactor. Circulation of the hydrocarbon synthesis slurry containing the fresh catalyst up through and out of the zone, and back into the slurry body in the synthesis reactor continues as long as the hydrogen flow up through the activity increasing zone is maintained. After the activity of the fresh catalyst is achieved, the hydrogen flow up through the catalyst activity increasing zone may be continued for catalyst rejuvenation. Thus, the same means and zone may be used for increasing the activity of the fresh catalyst and for catalyst rejuvenation. As a practical matter, the fresh catalyst activity increase occurs simultaneously with rejuvenation and it is preferred to remove at least a portion of the gas bubbles, which contain unreacted CO, from the slurry before it is passed into the activity increasing zone. To this extent, any means which permits rejuvenation while the synthesis reactor is producing hydrocarbons, such as those referred to above, can also be used for increasing the activity of the fresh catalyst and preferably those which remove gas bubbles from the slurry, before it is passed into the reaction zone in the means.
In one embodiment, the invention relates to a process for increasing the activity of a fresh hydrocarbon synthesis catalyst in a hydrocarbon synthesis slurry, which comprises (i) contacting the slurry with a hydrogen treat gas in one or more catalyst activity increasing reaction zones, to increase the activity of the fresh catalyst in the slurry, and (ii) passing the slurry containing the activity increased catalyst into a slurry body in a hydrocarbon synthesis reactor while the reactor is producing hydrocarbons from synthesis gas, wherein the slurry body comprises a hydrocarbon synthesis slurry comprising hydrocarbon synthesis catalyst particles and bubbles of synthesis gas in a hydrocarbon slurry liquid. Another embodiment relates to a slurry Fischer-Tropsch hydrocarbon synthesis process for producing hydrocarbons from a synthesis gas comprising a mixture of H2 and CO, wherein fresh synthesis catalyst is added to the hydrocarbon synthesis slurry and its activity is increased in-situ, in the hydrocarbon synthesis slurry, while the hydrocarbon synthesis reactor is producing hydrocarbons, by withdrawing a portion of hydrocarbon synthesis slurry from the slurry body and passing it and hydrogen or a hydrogen treat gas into one or more catalyst activity increasing reaction zones at least partially immersed in the slurry body in the synthesis reactor, in which the hydrogen contacts and increases the activity of the fresh catalyst. The slurry containing the activity increased catalyst passes out of the zone and back into the slurry body with which it mixes. It is preferred to remove at least a portion of the gas bubbles from the slurry before it enters the one or more activity increasing zones. It is more preferred to also remove at least a portion of any particulate solids present in the slurry, in addition to gas bubble removal, before it enters the one or more zones. In addition to unreacted synthesis gas, the gas bubbles also contain water vapor, hydrocarbon reaction products that are gaseous at the synthesis conditions, and inerts such as nitrogen and CO2. In a further embodiment, a portion of the hydrocarbon synthesis slurry containing fresh catalyst is circulated into and through one or more catalyst activity increasing zones external of the synthesis reactor, in which hydrogen contacts and increases the activity of the fresh catalyst particles in the slurry, with the slurry then returned back to the slurry body in the reactor. In a more specific embodiment, the invention is a slurry hydrocarbon synthesis process, which comprises:
(a) passing a synthesis gas comprising a mixture of H2 and CO up into a slurry body comprising a hydrocarbon synthesis slurry, in a hydrocarbon synthesis reactor, wherein the slurry comprises a particulate Fischer-Tropsch catalyst and bubbles of the synthesis gas dispersed in a hydrocarbon liquid, at reaction conditions effective for the H2 and CO in the synthesis gas to react and form hydrocarbons, at least a portion of which are liquid at the reaction conditions and which comprise the slurry hydrocarbon liquid;
(b) adding fresh catalyst particles to the hydrocarbon synthesis slurry, and
(c) passing a hydrogen treat gas and the hydrocarbon synthesis slurry containing the fresh catalyst particles through one or more catalyst activity increasing zones, in which the hydrogen in the treat gas contacts the fresh catalyst particles in the slurry, which increases the activity of at least a portion of them, wherein the slurry containing the activity increased catalyst exits the one or more zones and is passed into the slurry body in the reactor, and wherein the activity increase occurs while the synthesis reactor is producing hydrocarbons from the synthesis gas.
The passing and contacting will typically and preferably continue for a time sufficient to increase the activity of substantially all of the fresh catalyst particles added to the hydrocarbon synthesis slurry. Preferably at least a portion of the gas bubbles are removed from the slurry before it passes into the zone. Sufficient hydrogen or hydrogen treat gas is introduced into the zone to act as a lift gas and preferably provide slurry circulation into and through the activity increasing zone without the need for a mechanical slurry pump. The hydrocarbon slurry liquid produced in the reactor is separated from the catalyst particles either continuously or intermittently, and upgraded by one or more upgrading steps. These include one or more of fractionation and/or conversion operations, such as hydroconversion, in which the molecular structure of at least some the hydrocarbon molecules is changed. To the extent that the fresh catalyst is added to a hydrocarbon synthesis slurry containing reversibly deactivated catalyst particles, the activity increase of the fresh catalyst in the zone occurs simultaneously with catalyst rejuvenation. It has been found that both increasing the activity of fresh catalyst and catalyst rejuvenation will occur with CO present in the zone. However, the less the amount of CO present, the faster and more efficient will be the activity increase. Thus, if CO is present, the CO content of the total amount of the activity increasing hydrogen or hydrogen treat gas passed into the zone is preferably less than 10 mole % and the H2 to CO mole ratio in the gas is preferably greater than 3:1, more preferably greater than 4:1 and most preferably greater than 5:1. The offgas produced by the activity increasing or activity increasing and rejuvenation reactions in the zone will contain unreacted hydrogen. Those skilled in the art will appreciate that rejuvenation results in an activity increase of reversibly deactivated catalyst particles in the slurry. However, while there may be some overlap, in the context of the invention the term xe2x80x9cactivity increasexe2x80x9d is used to refer to fresh catalyst particles added to the slurry, while xe2x80x9crejuvenationxe2x80x9d refers to increasing the activity of reversibly deactivated catalyst particles.